When a newborn child is fully vaccinated with HBV and let's assume he's responsive, his antiHBs titers will be high. But since the antigenic stimulus is withdrawn, shouldn't the titre fall to nill in some time? Rather, the titers stay significant for around 40 more years. (>10 IU/L is significant)
So, how are there circulating antibodies for so long if there is no more antigenic stimulus?
I understand that memory cells have large half lives but what is producing the antibodies is the plasma cells which have very short half lives.
So how do the antibodies persist for so long?